We present the results of our experience in introducing modularity into the programming language Pascal in order to aid the creation and use of library modules. Our system performs the symbolic linking of source language modules producing a single Pascal text ready for compilation; performing the link phase before compilation anticipates interface consistency checks, and suggests a possible improvement of program development systems. Our extension is implemented in a preprocessor which ensures a complete compatibility with any standard Pascal compiler. In this paper we examine the main features of some high-level programming languages which support modularization and data abstraction and some experiences in introducing modularity into Pascal; on this basis we describe our choice in detail. The design and implementation details are discussed and some examples are presented.